1. Field of the Invention
The present invention relates to a process and apparatus for treating a polymer coated paperboard material. Specifically, the present invention relates to a process and an apparatus for double corona treating a polymer coated paperboard material.
2. Description of the Related Art
The fabrication of a paperboard material for certain packaging usually undergoes a process on a laminator where a polyethylene is extruded onto both surfaces of the paperboard material. While on the laminator, one or both surfaces of the paperboard material are subjected to a corona treatment. On certain grades of polymer-coated paperboard material, usually those with a reflective average (hereinafter "RA") of less that five which is mirror-like, it is very difficult to achieve an acceptable Dyne level from the corona treatment of the polymer-coated paperboard material. The smooth finish of the mirror pocket leaves the polyethylene very smooth making it difficult to print on the material at commercially acceptable levels. For comparison, a milk carton has a RA of 38 on the outside of the carton and 58 on the inside of the carton which are both relatively dull in comparison to a material having a RA of less than 5.
When processing a material with a RA of less than 5 on a laminator at a standard material speed of three hundred five meters per minute, a Dyne level of only thirty-eight Dyne per centimeter will be achieved on a conventional laminator. A Dyne level of at least forty-one Dyne per centimeter is required for printing at a commercially acceptable operating speed. In order to achieve such a Dyne level, the laminator would have to have its material speed reduced to less than one hundred eighty meters per minute, resulting in unacceptable (from a commercial perspective) laminator operating speeds. The Dyne level is a measurement of the surface tension of a material which is a mechanical force acting in the surface of the material.
Also, if the corona discharge has too great of power, the result will be pinholing in the material. Thus, there remains a need for producing a polymer-coated paperboard material with a Dyne level of at least 41 in a commercially viable manner.